Vacuum chuck for reducing distortion of semiconductor and GMR head wafers during processing

ABSTRACT

A flat vacuum chuck for restraining semiconductor wafer substrates and the like during processing applies a vacuum through holes in the chuck in a timed sequence. The localized areas of the wafer adjacent to each of the holes adhere to the chuck in a pattern that is controlled in such a way so as to minimize any residual gaps therebetween. In one application, the vacuum sequencing pattern is analogous to smoothing a curled sheet of paper on a flat surface with both hands by starting at or near the center of the sheet and moving both hands outward along the surface until the sheet is flat. The vacuum may be applied to substrates to overcome all types of distortions, including symmetrical, asymmetrical, and multi-plane distortions, depending upon the orientation of the net internal stresses resulting from the layers deposited on the wafer. The sequenced application of the vacuum through the chuck holes can be timed by installing a solenoid valve on each of the vacuum tubes connected to the ports. Alternatively, the vacuum applied through the chuck holes can be controlled by a single valve via different length tubes between the valve and the surface of the chuck. The holes with the shorter tubes reach nominal vacuum pressure sooner than those with longer tubes.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates in general to improved semiconductor waferprocessing, and in particular to improving flatness between asemiconductor wafer and a vacuum chuck. Still more particularly, thepresent invention relates to an improved apparatus and method forapplying a vacuum to a semiconductor wafer through a vacuum chuck toimprove flatness of the wafer during processing.

2. Description of the Related Art

As semiconductor or giant magneto resistive (GMR) head wafers areprocessed, various metallic and non-metallic layers are deposited on oneside of the original wafer substrate. Even if the original substrate isvery flat, the deposition of material layers with residual tensile orcompressive stresses on only one side of the wafer will cause the entirewafer to distort from its original state of flatness in directionsnormal to the plane of the wafer. Because of this distortion, there is aneed for the wafer to be held and restrained as flat as possible duringthe various process steps, such as photoresist application and exposure,material deposition, and metrology. Photoresist exposure processing isparticularly sensitive to flatness since non-flat deviations can lead tolocation errors of the features being printed on the wafer, andoversized or undersized feature printing.

Prior art process machines utilize a vacuum chuck to hold the wafer flatduring processing. A vacuum chuck is a platform that may be larger,smaller, or the same size as the wafer. The wafer is placed on the chuckby an operator or a robot with the non-processed side of the waferfacing downward against the chuck. Typically, the surface of the chuckis held to an extremely tight flatness tolerance, although non-flatshapes such as spherical or cylindrical shapes also can be used. Theface of the vacuum chuck facing the wafer usually has many open holesthrough which a vacuum is applied. The vacuum originates from an outsidevacuum source via tubes or tubular ports inside the chuck. Duringprocessing, the vacuum holds the wafer in place on the chuck. Afterprocessing, the vacuum is discontinued so that the wafer may be removedfrom the chuck.

Most prior art process machines apply the vacuum to all of the holes inthe face of the chuck at the same time. For example, FIG. 1 depicts aflat, circular vacuum chuck 11 with a plurality of vacuum holes 13arrayed across its surface. The small numerals located adjacent to thelower right side of each of the holes 13, schematically illustrate thetiming sequence of the vacuum applied to the holes 13. In FIG. 1, sinceeach of the holes 13 has the same numeral “1”, the vacuum is applied toeach of the holes 13 at the same time.

However, since neither the wafers nor the vacuum chucks are perfectlyflat, a problem arises with this prior art method as the vacuum isapplied simultaneously across all of the holes. The localized areas onthe wafer where the gaps between the wafer and the chuck are smallestwill adhere to the chuck first. Other areas on the wafer, where the gapsbetween the wafer and the chuck are larger, will not be pulled tightlyagainst the chuck due to static friction between the wafer and the chuckat the earlier adhesion areas. As a result, tiny gaps remain between thewafer and the chuck such that the wafer does not completely deform tothe flat shape of the chuck. These gaps of separation can be even largerwhen the wafer substrates are made from rigid materials such as thetitanium carbide used to manufacture GMR heads.

U.S. Pat. No. 5,094,536 discloses a deformable vacuum chuck withdistorting actuators that may be manipulated to directly compensate fordistortions in a wafer. This is a very elaborate and complicatedmechanical apparatus that requires feedback from an interferometersystem. U.S. Pat. No. 5,564,682 discloses a sequenced vacuum method forcorrecting symmetrical, single-plane wafer distortions around a linethat is perpendicular to the wafer and through its center.Unfortunately, this reference has very limited application since manywafer distortions are asymmetrical and/or exist in multiple planes.Thus, an improved apparatus and method for improving flatness between asemiconductor wafer and vacuum chuck is needed.

SUMMARY OF THE INVENTION

A flat vacuum chuck for restraining semiconductor wafer substrates andthe like during processing applies a vacuum through holes in the chuckin a timed sequence. The localized areas of the wafer adjacent to eachof the holes adhere to the chuck in a pattern that is controlled in sucha way so as to minimize any residual gaps therebetween. In oneapplication, the vacuum sequencing pattern is analogous to smoothing acurled sheet of paper on a flat surface with both hands by starting ator near the center of the sheet and moving both hands outward along thesurface until the sheet is flat. The vacuum may be applied to substratesto overcome all types of distortions, including symmetrical,asymmetrical, and multi-plane distortions, depending upon theorientation of the net internal stresses resulting from the layersdeposited on the wafer. The sequenced application of the vacuum throughthe chuck holes can be timed by installing a solenoid valve on each ofthe vacuum tubes connected to the ports. Alternatively, the vacuumapplied through the chuck holes can be controlled by a single valve viadifferent length tubes between the valve and the surface of the chuck.The holes with the shorter tubes reach nominal vacuum pressure soonerthan those with longer tubes.

Accordingly, it is an object of the present invention to provideimproved semiconductor wafer processing.

It is an additional object of the present invention to provide improvedflatness between a semiconductor wafer and a vacuum chuck.

Still another object of the present invention is to provide an improvedapparatus and method for applying a vacuum to a semiconductor waferthrough a vacuum chuck to improve flatness of the wafer duringprocessing.

The foregoing and other objects and advantages of the present inventionwill be apparent to those skilled in the art, in view of the followingdetailed description of the preferred embodiment of the presentinvention, taken in conjunction with the appended claims and theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the features, advantages and objects of theinvention, as well as others which will become apparent, are attainedand can be understood in more detail, more particular description of theinvention briefly summarized above may be had by reference to theembodiment thereof which is illustrated in the appended drawings, whichdrawings form a part of this specification. It is to be noted, however,that the drawings illustrate only a preferred embodiment of theinvention and is therefore not to be considered limiting of its scope asthe invention may admit to other equally effective embodiments.

FIG. 1 is a schematic diagram of a prior art vacuum pattern for asemiconductor wafer vacuum chuck.

FIG. 2a is a schematic top view of a first embodiment of a vacuumpattern for a semiconductor wafer vacuum chuck and is constructed inaccordance with the invention.

FIG. 2b is a schematic sectional side view of the chuck in FIG. 2a takenalong the line 2 b—2 b and shown with a distorted wafer.

FIG. 2c is a schematic sectional end view of the chuck in FIG. 2a takenalong the line 2 c—2 c and shown with a distorted wafer.

FIG. 3a is a schematic top view of a second embodiment of a vacuumpattern for the semiconductor wafer vacuum chuck of FIG. 2a.

FIG. 3b is a schematic sectional side view of the chuck in FIG. 3a takenalong the line 3 b—3 b and shown with a distorted wafer.

FIG. 3c is a schematic sectional end view of the chuck in FIG. 3a takenalong the line 3 c—3 c and shown with a distorted wafer.

FIG. 4 is a schematic top view of a third embodiment of a vacuum patternfor the semiconductor wafer vacuum chuck of FIG. 2a.

FIG. 5a is a schematic top view of a fourth embodiment of a vacuumpattern for the semiconductor wafer vacuum chuck of FIG. 2a.

FIG. 5b is a schematic sectional side view of the chuck in FIG. 5a takenalong the line 5 b—5 b and shown with a distorted wafer.

FIG. 5c is a schematic sectional side view of the chuck in FIG. 5a takenalong the line 5 c—5 c and shown with a distorted wafer.

FIG. 6a is a schematic top view of a fifth embodiment of a vacuumpattern for the semiconductor wafer vacuum chuck of FIG. 2a.

FIG. 6b is a schematic sectional side view of the chuck in FIG. 6a takenalong the line 6 b—6 b and shown with a distorted wafer.

FIG. 6c is a schematic sectional end view of the chuck in FIG. 6a takenalong the line 6 c—c and shown with a distorted wafer.

FIG. 7a is a schematic top view of a sixth embodiment of a vacuumpattern for the semiconductor wafer vacuum chuck of FIG. 2a.

FIG. 7b is a schematic sectional side view of the chuck in FIG. 7a takenalong the line 7 b—7 b and shown with a distorted wafer.

FIG. 7c is a schematic sectional end view of the chuck in FIG. 7a takenalong the line 7 c—7 c and shown with a distorted wafer.

FIG. 8a is a schematic top view of a seventh embodiment of a vacuumpattern for the semiconductor wafer vacuum chuck of FIG. 2a.

FIG. 8b is a schematic sectional side view of the chuck in FIG. 8a takenalong the line 8 b—8 b and shown with a distorted wafer.

FIG. 8c is a schematic sectional end view of the chuck in FIG. 8a takenalong the line 8 c—8 c and shown with a distorted wafer.

FIG. 9 is a schematic sectional side view of a vacuum chuck with a firstversion of a mechanism for sequencing a vacuum across the chuck.

FIG. 10 is a schematic sectional side view of a vacuum chuck with analternate version of a mechanism for sequencing a vacuum across thechuck.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As described in the Background section of this disclosure, semiconductorwafers and the like become distorted as various metallic andnon-metallic layers are deposited on one side of the original wafersubstrate during processing. Even if the original substrate is veryflat, the deposition of material layers with residual tensile orcompressive stresses on only one side of the wafer will cause the entirewafer to distort from its original state of flatness in directionsnormal to the plane of the wafer.

When the net internal stresses applied by the layers deposited on thewafer are compressive, the wafer typically bows in a generally “concave”manner. To prevent residual gaps between a concave wafer and the chuck,the vacuum timing sequences of embodiments of FIGS. 2-4 of the inventionare suggested. When the net internal stresses applied by the layersdeposited on the wafer are tensile in nature, the wafer typically bowsin a generally “convex” manner. To prevent residual gaps between aconvex wafer and the chuck, the vacuum timing sequences of embodimentsof FIGS. 5-8 of the invention are suggested. These embodiments aredescribed in detail below.

Referring to FIGS. 2a, 2 b, and 2 c, a first embodiment of a system andmethod for processing a subject workpiece such as a semiconductor wafersubstrate or the like is depicted as a vacuum chuck 21. Chuck 21 has anarray of a plurality of ports or holes 23 which extend through it. Inthe version shown, chuck 21 is a hard, flat, circular surface with 29holes 23. At least some of the holes 23 lie along a lateral axis ofchuck 21 (i.e., horizontally, or left to right), and some of the holes23 lie along a transverse axis of chuck 21 (i.e., vertically, or top tobottom). However, chuck 21 may comprise many different shapes and sizeswith more or fewer holes 23, depending upon the application.

A vacuum is drawn through holes 23 via a vacuum source 25 (representedby the arrows in FIGS. 2b and 2 c) in order to pull a distorted wafer 27tight against the flat chuck 21. In FIGS. 2b and 2 c, the curvature ordistortion of wafer 27 is greatly exaggerated to better illustrate theadvantages of the invention. The small, single digit numerals locatedadjacent to the lower right of each of the holes 23 schematicallyillustrate the timing sequence of the vacuum applied to the holes 23.The vacuum is first drawn through the centermost hole 23 which has thenumeral “1.” The two holes 23 located directly above and below thecentermost hole 23 (FIG. 2a) are labeled “2” since the vacuum is appliedto each of these holes 23 shortly after the vacuum is applied to thecentermost hole 23. Next, the timing sequence radially progresses to thetwo holes 23 that are labeled “3,” and so on. In FIG. 2a, the smallarrows and the dashed lines joining respective ones of thelike-enumerated holes 23 are provided for graphically illustrating thetiming sequence for the vacuum.

The first embodiment of FIGS. 2a—2 c is suited for correctingasymmetrical distortion of a wafer that is only distorted in one plane,such as the uniformly cross-sectioned, arcuate or concave wafer 27 inFIGS. 2b and 2 c. Wafer 27 only distorts or “bows” in the plane anddirection shown in FIG. 2b; it does not bow in any other direction asshown by the flat cross-sectional shape in FIG. 2c. The vacuum timingsequence illustrated and described above initially draws wafer 27 tightagainst chuck 21 where the gap between them is consistent and smallest(i.e., the central width or “spine” of wafer 27 shown in FIG. 2c; e.g.,vertically). The system then progressively or incrementally evacuatesthe surrounding columns of holes 23 (FIG. 2a) such that thenext-smallest gap between wafer 27 and chuck 21 is minimized (the holes23 enumerated “5” in FIGS. 2a and 2 b), and so on. Sequencing the vacuumin this manner ensures that any residual gaps between chuck 21 and wafer27 are overcome and minimized.

Referring now to FIGS. 3a-3 c, a second embodiment of a system andmethod for processing a wafer is shown as a vacuum chuck 31. Like chuck21, chuck 31 has an array of vacuum holes 33 extending therethrough. Thevacuum is drawn through holes 33 via vacuum source 35 to pull adistorted wafer 37 tight against chuck 31. As described above, thesmall, single digit numerals located adjacent to the lower right of eachhole 33 schematically illustrate the timing sequence of the vacuumapplied to holes 33.

In this version, the vacuum is first drawn through the hole immediatelyadjacent to the right side of the centermost hole 33, labeled “1.” Theeight, square-arrayed holes 33 labeled “2” immediately surrounding thehole 33 labeled “1” (FIG. 3a) are evacuated shortly thereafter in agenerally concentric pattern. The timing sequence progresses to thearray of twelve holes 33 labeled “3,” and so on. In FIG. 3a, the dashedlines joining respective ones of the like-enumerated holes 33 assist ingraphically illustrating the timing sequence of the vacuum.

This second embodiment is ideally suited for correcting the asymmetricaldistortion of a wafer that is distorted in two planes, such as the(greatly exaggerated) concave wafer 37 in FIGS. 3b and 3 c. Wafer 37bows in each of the planes and directions shown in FIGS. 3b and 3 c. Thevacuum timing sequence initially draws wafer 37 tight against chuck 31at the smallest gap near the center. The system then progressivelyevacuates the surrounding holes 33 in numerical order such that thenext-smallest gaps between wafer 37 and chuck 31 are minimized, and soon, until the entire 37 is pulled tight against chuck 31.

Referring now to FIG. 4, a third embodiment of a system and method forprocessing a wafer is shown as a vacuum chuck 41. Like chuck 31, chuck41 is ideally suited for correcting biplanar distortion of a wafer, suchas the concave wafer 37 illustrated in FIGS. 3b and 3 c. Chuck 41 has anarray of vacuum holes 43 with small numerals located adjacent to thelower right of each hole 43 for schematically illustrating the timingsequence of the vacuum applied to holes 43.

In this version, the vacuum is first drawn through the near-center hole43, labeled “1” (FIG. 4). The four, diamond-arrayed holes 43 labeled “2”immediately surrounding the near-center hole 43 are evacuated shortlythereafter. The timing sequence concentrically progresses to the four,square-arrayed holes 43 labeled “3,” and so on to the perimeter hole 43labeled “9.” The dashed lines joining respective ones of thelike-enumerated holes 43 assist in graphically illustrating the vacuumsequence. The vacuum timing sequence initially draws the wafer tightagainst chuck 41 at the smallest gap near the center. The system thenprogressively evacuates the surrounding holes 43 in numerical order suchthat the next-smallest gaps between the wafer and chuck 41 areminimized.

Referring now to FIGS. 5a and 5 b, a fourth embodiment of a system andmethod for processing a wafer is shown as a vacuum chuck 51. Chuck 51 isdesigned to correct an asymmetrical, convex distortion in a wafer, suchas wafer 57 in FIG. 5b. The asymmetrical distortions in wafer 57 renderits exaggerated appearance as somewhat like the lemniscatic undulationsof a potato chip. Chuck 51 has an array of enumerated vacuum holes 53for schematically illustrating the timing sequence of the vacuum 55applied to holes 53. In this version, the vacuum is first drawn throughthe leftmost hole 53, labeled “1.” The remaining holes 53 are evacuatedin the enumerated order, essentially moving from the center outward andfrom left to right, one set after another. The dashed lines joiningrespective ones of the like-enumerated holes 53 in FIG. 5a helpillustrate the generally columnar sequence. The vacuum timing sequenceinitially draws wafer 57 tight against chuck 51 at the smallest gap onthe center left before progressively evacuating the other holes 53 innumerical order such that the gaps between wafer 57 and chuck 51 areminimized.

A fifth embodiment of the invention is shown as a vacuum chuck 61 inFIGS. 6a—6 c. Chuck 61 is designed to correct convex distortion in awafer, such as wafer 67 in FIGS. 6b and 6 c. Chuck 61 has an array ofenumerated vacuum holes 63 for schematically illustrating the timingsequence of the vacuum 65 applied thereto. In this version, the vacuumis first drawn through the leftmost hole 63, labeled “1.” The remainingholes 63 are evacuated in the enumerated order, again essentially movingfrom the center outward and from left to right. The dashed lines andarrows in FIG. 6a help illustrate the sequence. The vacuum timingsequence draws wafer 67 tight against chuck 61 at the smallest gap onthe center left and progressively evacuates the other holes 53 insequence to minimize the gaps between wafer 67 and chuck 61.

In FIGS. 7a-7 c, a sixth embodiment of the invention is shown as avacuum chuck 71. Chuck 71 corrects convex distortion in a wafer, such aswafer 77 in FIGS. 7b and 7 c. Chuck 71 has an array of enumerated vacuumholes 73 which illustrate the timing sequence of the vacuum 75 appliedthereto. The vacuum is initially drawn through the leftmost hole 73,labeled “1” and proceeds directly to the right to the holes 73 labeled“2”, “3”, and “4.” After the holes 73 labeled “4” are evacuated, twosets of horizontal rows of holes 73 are evacuated as represented atnumerals “5” and “6.” Next, the vacuum is applied to the two perimeterholes 73 enumerated “7,” before the two vertical columns of holes 73labeled “8” and “9” are sequentially evacuated. The dashed lines andarrows in FIG. 7a illustrate this sequence.

Another embodiment of the invention is shown as a vacuum chuck 81 inFIGS. 8a-8 c. Chuck 81 is designed to correct convex distortion in awafer, such as wafer 87 in FIGS. 8b and 8 c. Chuck 81 has an array ofenumerated vacuum holes 83 which illustrate the timing sequence of thevacuum 85 applied thereto. The vacuum is initially drawn through theleftmost hole 83, labeled “1.” After hole 83 “2” is evacuated, anarrowhead pattern is established beginning with the three holes 83labeled “3.” The arrowhead pattern continues with hole 83 sets “4”through “7” before the two holes “8” are evacuated. The dashed lines andarrows in FIG. 8a help illustrate the sequence. The vacuum timingsequence draws wafer 87 tight against chuck 81 at the smallest gap onthe center left and progressively evacuates the other holes 83 insequence to minimize the gaps between wafer 87 and chuck 81.

Referring now to FIG. 9, one apparatus and method for implementing thepreviously described vacuum timing sequences is shown. FIG. 9illustrates a vacuum chuck 91 having an array or plurality of holes 93that extend therethrough to its upper surface. The lower end of eachhole 93 is interconnected to a conduit with a valve 95, such as asolenoid valve, mounted thereto. The conduits and valves 95 areconnected in parallel to a vacuum pump 97 for drawing a vacuum throughholes 93 in order to secure an object such as a wafer 98. Both thevalves 95 and vacuum pump 97 are controlled via a controller 99, such asa microprocessor. Controller 99 sequences the opening of valves 95 suchthat any of the previously described evacuation patterns may beaccomplished in order to overcome many different types of distortion inwafer 98.

Another apparatus and method for implementing the previously describedvacuum timing sequences is shown in FIG. 10. Here, a vacuum chuck 101has an array through-holes 103 that extend to its upper surface. Thelower end of each hole 103 is interconnected to a manifold through asingle valve 105. The manifold and valve 105 are connected to a vacuumpump 107 for drawing a vacuum through holes 103 to secure an object suchas a wafer 108. Note that the different tubes comprising the manifoldvary in length. For example, tube 104 is significantly longer than tube106, thus permitting tube 106 and its respective hole 103 to beevacuated prior to tube 104 and its respective hole 103. The simplevariance in tube length allows an operator to sequence the vacuumapplied to holes 103 as needed. Both the valve 105 and vacuum pump 107are controlled via a controller 109, such as a microprocessor.Controller 109 sequences the opening of valve 105 such that any of thepreviously described evacuation patterns may be accomplished in order toovercome many different types of distortion in wafer 108.

The invention has several advantages including the ability toindependently control each of the holes in a vacuum chuck to correctdistortions that are not symmetrical about a line normal to the waferand through its center. In other words, the distorted wafer does nothave to look like a “perfect bowl.” Moreover, the invention is capableof minimizing residual gaps between wafers having a variety ofdistortions and the chuck so that process variations across each wafer,and from one wafer to the next wafer are minimized. When processvariation is minimized, the process mean can be more accurately moved tothe nominal condition to improve yields and, hence, lower manufacturingcosts. In GMR head wafer processes such as photoresist exposure, flatwafers are essential for achieving sub-half-micron pole tip and GMRsensor dimensions. This system and method is capable of reducingsymmetrical and non-symmetrical distortions such as non-flat sphericaland/or cylindrical shapes. The requirement of a microprocessor in thefirst of the two method for implementing sequenced vacuum patterns isnot a drawback since a microprocessor is typically required in waferprocessing equipment to control movement of other parts of the machine.

While the invention has been shown or described in only some of itsforms, it should be apparent to those skilled in the art that it is notso limited, but is susceptible to various changes without departing fromthe scope of the invention. For example, other timing sequences arepossible to nullify other types of wafer distortion patterns.

What is claimed is:
 1. An apparatus for overcoming distortions in aworkpiece, comprising: a vacuum chuck having a surface, a perimeter, andan array of ports extending through the surface inside the perimeter,the surface being adapted to support a workpiece thereon; a vacuumsource for drawing a vacuum through the array of ports such that theworkpiece is adapted to adhere to the surface of the vacuum chuck;control means for selectively evacuating the array of ports in thevacuum chuck via the vacuum source in a timed sequence so as to overcomeasymmetrical distortion in the workpiece; and wherein the ports arearranged in multiple, symmetrical, concentric square rings centered on acentral port located at an intersection of perpendicular surface axes,with at least some of the ports aligned with each of the axes, and aplurality of outlying ports aligned with the axes and located radiallybeyond an outermost square ring adjacent to a perimeter of the chuck. 2.The apparatus of claim 1 wherein the control means selectively evacuatesthe array of ports generally from approximately a center of theworkpiece to two perimeter sides of the workpiece, and fromapproximately the center of the workpiece to two lateral perimeter sidesof the workpiece.
 3. The apparatus of claim 1 wherein the control meansselectively evacuates the array of ports in a generally radial patternfrom approximately a center of the workpiece to the perimeter of theworkpiece.
 4. The apparatus of claim 1 wherein the control meansselectively evacuates the array of ports generally from one side of theworkpiece to an opposite side of the workpiece.
 5. The apparatus ofclaim 1 wherein the control means selectively evacuates the array ofports generally from approximately a center of a first side of theworkpiece radially outward to a second side of the workpiece.
 6. Theapparatus of claim 1 wherein the control means selectively evacuates thearray of ports generally from approximately a center of a first side ofthe workpiece radially inward to approximately a center of theworkpiece, and radially outward to the perimeter of the workpiece. 7.The apparatus of claim 1 wherein the control means selectively evacuatesthe array of ports generally from approximately a center of a first sideof the workpiece radially across the workpiece to approximately a centerof an opposite side of the workpiece, and radially outward in twodirections to the perimeter of the workpiece.
 8. An apparatus forovercoming distortions in a workpiece, comprising: a vacuum chuck havinga planer surface with a lateral axis extending from a first lateral sideto a second and opposite lateral side, and a transverse axisperpendicular to the lateral axis extending from a first transverse sideto a second and opposite transverse side, and an array of portsextending through the surface with at least some of the ports beinggenerally located along each of the axes, the surface being adapted tosupport a workpiece thereon; a vacuum source for drawing a vacuumthrough the array of ports such that the workpiece is adapted to adhereto the surface of the vacuum chuck; a controller for selectivelyevacuating the array of ports in the vacuum chuck via the vacuum sourcein a timed sequence so as to overcome asymmetrical distortion in theworkpiece; and wherein the ports are arranged in multiple, symmetrical,concentric, square rings centered on a central port located at anintersection of the axes, and a plurality of outlying ports aligned withthe axes and located radially beyond an outermost square ring adjacentto a perimeter of the chuck.
 9. The apparatus of claim 8 wherein thecontroller selectively evacuates the array of ports generally from acenter of the workpiece to each of the transverse sides of theworkpiece, and in columns from approximately the center of the workpieceto each of the lateral sides of the workpiece.
 10. The apparatus ofclaim 8 wherein the controller selectively evacuates the array of portsin a generally radial pattern from approximately a center of theworkpiece simultaneously to both the lateral and transverse sides of theworkpiece in a concentric pattern.
 11. The apparatus of claim 8 whereinthe controller selectively evacuates the array of ports generally fromthe first lateral side of the workpiece to the second lateral side ofthe workpiece in columns while expanding to each of the transverse sidesof the workpiece.
 12. The apparatus of claim 8 wherein the controllerselectively evacuates the array of ports generally from approximately acenter of the first lateral side of the workpiece radially across theworkpiece in columns to the second lateral side of the workpiece andradially outward to each of the transverse sides of the workpiece. 13.The apparatus of claim 8 wherein the controller selectively evacuatesthe array of ports generally from approximately a center of the firstlateral side of the workpiece radially inward to approximately a centerof the workpiece, radially outward to each of the transverse sides ofthe workpiece in columns, and laterally across the workpiece in columnsfrom approximately the center of the workpiece to the second lateralside of the workpiece.
 14. The apparatus of claim 8 wherein thecontroller selectively evacuates the array of ports generally from acenter of the first lateral side of the workpiece radially across theworkpiece to approximately a center of the second lateral side of theworkpiece, and radially outward toward both transverse sides of theworkpiece in an arrowhead pattern.
 15. An apparatus for overcominglemniscatic distortions in a workpiece, comprising: a vacuum chuckhaving a planer surface that is adapted to support the workpiece, thesurface having a lateral axis extending from a first lateral side to asecond and opposite lateral side, and a transverse axis perpendicular tothe lateral axis extending from a first transverse side to a second andopposite transverse side; ports extending through the surface andarrayed in multiple, symmetrical, concentric, square rings centered on acentral port located at an intersection of the axes, with at least someof the ports aligned with each of the axes, and a plurality of outlyingports aligned with the axes and located radially beyond an outermostsquare ring adjacent to a perimeter of the chuck; a tube coupled to andextending from each of the ports, wherein the tubes vary in length tovary their sequence of evacuation; a tube manifold interconnected toeach of the tubes and having a valve; a vacuum source coupled to thetube manifold for drawing a vacuum through the ports such that theworkpiece adheres to the surface of the vacuum chuck; and a controllerfor selectively evacuating the ports via the vacuum source inasymmetrical, off-center patterns to correct lemniscatic undulation inthe workpiece.